Global navigation satellite systems (GNSS) allow electronic receivers to determine navigational information such as position (latitude, longitude, and altitude), velocity and time, also known as PVT information. One example of such a system is the United States Naystar Global Positioning System (GPS), which may include up to thirty-two or more functional navigation satellites. Other examples of satellite navigation systems include the Russian GLONASS system, the European Galileo system, the Chinese Compass/Beidou system, and the Japanese QZSS. Satellite navigation receivers, such as GPS receivers typically use GPS data from three or more orbiting satellites to determine navigation information. Only a portion of the satellites within a navigation system may be visible to a particular navigation receiver at a given time.
GPS satellites typically transmit a signal based on spread spectrum coding known as code division multiple access (CDMA) on two bands, the L1 band with a carrier frequency of 1575.42 MHz and the L2 band with a carrier frequency of 1227.60 MHz. Each satellite is assigned a coarse acquisition (C/A) code (PRN) that resembles pseudo random noise and is typically unique to that satellite. Once a GPS signal with a particular PRN code is received and identified, the GPS receiver is said to have “acquired” the GPS satellite associated with that PRN. A GPS receiver may also “track” a GPS satellite by continuing to receive a GPS signal from a previously acquired GPS satellite.
The conventional approach to using GPS satellites for user positioning requires the receiver to acquire, track and download the navigation message from 4 or more visible satellites in order to determine an adequate PVT solution. The navigation message from each satellite contains the broadcast ephemeris, the ionospheric models, and UTC-GPS clock correction that are necessary for the user to compute the position of the satellites in the earth-centered earth-fixed (ECEF) coordinate system for a specified time. Given that satellite signals are relatively weak, it can be difficult to maintain adequate signal reception for the period of time necessary to download the required information. These problems are exacerbated when external conditions interfere with signal reception, such as when the receiver is indoors or in environments having tall buildings (an “urban canyon”), obscuring topographical features or dense foliage.
To mitigate these problems, techniques to supplement the information delivered by satellite have been developed, and are generally known as Assisted GNSS, A-GPS for example. Such techniques involve the delivery of ephemeris or other data over a network rather than being received from a satellite. Since it is not necessary to wait for download of the broadcast ephemeris data, performance metrics such as time to first fix (TTFF), reliability and receiver sensitivity can be dramatically improved, particularly when the sub-optimal signal conditions associated with urban canyons or indoor locations interfere with the download of the complete satellite broadcast. Use of other sources of navigational data can also minimize power consumption, since less time is required to maintain a fix on the satellites and a position determination can be made more quickly.
The performance of a GNSS receiver also depends heavily on its ability to rapidly find visible satellites. Acquisition of a satellite involves correlating the incoming signal with a local code replica, which is characterized with particular values for the frequency offset to account for Doppler variations and code phase delay. A traditional approach is to sequentially scan all possible combinations of frequencies and code-phases in the search space, until the correlation value exceeds a predefined threshold. If an insufficient number of satellites are acquired during the search at a given signal level, the function is then repeated using parameters for the next sequential level of search. Although such techniques will generally acquire satellites having sufficient signal strength, there is a considerable time overhead to exhaust all combinations at all signal levels.
As will be appreciated, GNSS receivers are expected to operate quickly in a wide variety of operating conditions. For example, mobile cellular telephones are required to meet certain Enhanced 911 (E911) standards to provide emergency responders with accurate location information under many signal conditions, including strong signals, weak signals and combinations of strong and weak signals. To meet these standards, an associated GNSS receiver must be able find both strong and weak signals relatively quickly. Conventional search strategies that simply sequentially increase the sensitivity of the receiver will often fail to find weak signals within the required time limits. Similarly, search strategies that focus on weak signals first often do not provide sufficient performance with respect to strong signals.
Accordingly, it would be desirable to provide a mobile receiver that provides improved performance in mixed signal conditions, including having lower TTFFs. This invention satisfies these and other needs.